BRCA1 is a multi-functional breast and ovarian cancer suppression gene that is suspected of operating in this manner by suppressing the development of genome disorder. But how it delivers signals that elicit a clinical cancer suppressing effect is unknown. During the current granting period, we reported that the BRCA1 protein (aka B1), itself, executes at least one of its multiple biochemical functions under tight control afforded bya B1-bound, multi-subunit protein complex, called RAP80. This larger complex is called the RAP80/B1 complex, and it allows B1 to perform at normal amplitude error-free double strand DNA break (DSB) repair by homologous recombination(HR). Thus, RAP80/B1 physiologically regulates B1 HR function, which is a known contributor to B1 cancer suppression. This process is termed `HR tuning'. We have also reported that there is another protein present in RAP80/B1 complexes, i.e. the enzyme PARP1. In these complexes, PARP1 selectively poly-ADP ribosylates (parsylates) the DNA binding domain of B1 to form a parsylated derivative, aka B1pars. RAP80/B1 complex formation and proper B1 parsylation contribute to B1 HR and B1 genome stability control. All of these events take place in and require the operation of specialized, DSB- containing nuclear focal structures (aka IRIF) that form after DNA damage. In this proposal, we will determine: a) whether PARP1-catalyzed B1 parsylation regulates any of the other, known functions of B1, including cancer suppression; b) how the specific PARP1-driven parsylation of B1 is directed to its DNA binding domain; c) whether defects in B1 parsylation and RAP80/B1pars complex formation elicit clinically important contributions to sporadic breast and ovarian cancer development; and d) whether they dictate a specific strategy for achieving effective breast and ovarian cancer therapy.